1. Technical Field
The present invention relates to an organic EL device which is used in a display, a display light source, and the like, and more specifically relates to a compound for an organic EL device which is preferably used in a coated organic EL device, and an organic EL device using the same.
2. Related Art
In recent years, development of organic electroluminescent devices (also referred to organic EL devices) which serves as self-emissive type displays replacing liquid crystal displays, has been rapidly advancing. Such an organic EL device and a method of preparing the same have been disclosed in, for example, JP-A-2000-323276, JP-T-2002-536492, JP-A-63-264692, and JP-A-2003-40845.
In the related art, for an organic EL device (also referred to an organic EL device), technologies involving using a host material and a light-emitting dopant for the material configuration for a light-emitting layer in order to accomplish higher luminous efficiency, change an luminescent colors, and longer life time, are known. Such technologies are frequently used particularly for a device in which an organic material in an organic EL device is applied using a deposition method, but they have been rarely used for a polymeric material-coated organic EL device in which a film is formed by using an ink jet method (liquid droplet discharge method) or a spin coating method.
As used herein, the phrase “the host material and the light-emitting dopant” are defined/characterized as follows:
(1) The host material refers to a material capable of transporting both holes and electrons.
(2) For an organic EL device in which a light-emitting dopant is not used in a light-emitting layer, light emission from a host material is observed. On the other hand, for an organic EL device in which both a light-emitting dopant and a host material are used, light emission from the host material is substantially not observed, but light emission from the light-emitting dopant is observed in most cases.
(3) The EL light-emitting spectrum observed in an organic EL device in which both a host material and a light-emitting dopant are used is that of fluorescent light or phosphorescent light of a light-emitting center in the light-emitting dopant. As used herein, the phrase “light-emitting center” refers to a part of a light-emitting dopant. It means an organic molecule backbone capable of emitting a strong fluorescent light/phosphorescent light, and also means a partial backbone by which the waveform of light is substantially determined.
For the polymeric material-coated organic EL device, the reasons why technologies involving using both a host material and a light-emitting dopant have been rarely used are presumably as follows.
(1) When a mixed solution of a host and a light-emitting dopant is coated, a phenomenon in which the light-emitting dopant bleeds out is observed. This is generally due to the fact that there exists a phenomenon that when a mixed solution of a low molecular weight material in a polymer is coated and dried, the low molecular weight material is exuded to an outer layer, or it becomes segregated from a coated film during drying.
(2) The light-emitting dopant material has little capability of trapping holes/electrons. This is prominent in the case where the host is a conjugated polymer. In the case where the host is a conjugated polymer, holes and electrons preferentially flow within the host polymer molecule. Thus, it is expected that it is difficult for a light-emitting dopant to trap holes and electrons.
(3) Development of a light-emitting dopant material has proceeded slowly. It is believed that due to the aforementioned two reasons, in the field of polymer-coated organic EL devices, a system composed of a host plus a light-emitting dopant has a lower effect than a deposition-based EL (low molecular weight EL), and thus development of such materials has proceeded slowly.
An advantage of some aspects of the invention is that it provides a compound for an organic EL device, which functions as a light-emitting dopant in a light-emitting layer, as a material for forming light-emitting layer, particularly capable of exhibiting higher luminous efficiency and longer life time, and an organic EL device using the same. Specifically, the compound for an organic EL device and the organic EL device according to some aspects of the invention are provided for an organic EL device which emits light in a luminescent color region determined by a light-emitting molecule, and exhibits improved luminous efficiency and luminance half life.
The present inventors have conducted extensive studies in order to solve the above-described problems, and as a result, they have found the following knowledge.
In order to solve the problem as described in (1) the following means can be employed. By increasing the molecular weight of the light-emitting dopant, the light-emitting dopant would not bleed out during applying and drying. It is preferable that the entire dopant preferably includes a conjugated polymer having N atoms, in addition to a general π conjugated polymer. However, in a coated organic EL, it is necessary to dissolve the dopant in a suitable solvents, and thus according to the design, its molecular weight may be increased with a non-conjugated linkage to a suitable molecular weight.
Further, in order to solve the problem as described in (2), the following means can be employed. When the molecular weight is increased for the purpose of solving the problem as described in (1), a functional group capable of preferentially trapping holes or electrons is incorporated in the light-emitting dopant molecule to improve the function of the dopant, as a guideline for the molecular design. Particularly, a functional group having a hole trapping function is incorporated in the molecule to improve the function as a dopant.
It is preferable that the light-emitting center in the dopant and the functional group having a hole trapping function are connected to each other through π-conjugation. This is because the same effect as the case where the light-emitting center directly traps holes can be attained by π-conjugation.
One of the measures for improving a hole trapping property is an IP value (ionization potential) of a host material, which is required to be equal to or more than the oxidation potential of the host material.
Further, the present inventors have continued extensive studies based on these findings, thereby completing the invention.
That is, the compound for an organic EL device according to an aspect of the invention is a compound for an organic EL device as a light-emitting material for use in an organic EL device, comprising polymer molecules containing light-emitting molecules for determining a luminescent color region of the light-emitting material, and molecules represented by Formulas (1) to (4) as constituent units.
(wherein R represents an alkyl group, an aryl group, or an alkylaryl group)
(wherein R represents hydrogen, an alkyl group, or an alkylaryl group)
Further, in the compound for an organic EL device, the light-emitting molecule preferably contains one molecule selected from the molecules represented by Formulas (5) to (7).

In the polymer molecules, the light-emitting molecule represented by Formula (5) functions as a molecule unit which emits yellow light in an organic EL device. The light-emitting molecule represented by Formula (6) functions as a molecule unit which emits yellow light in an in an organic EL device. Further, the light-emitting molecule represented by Formula (7) functions as a molecule unit which emits green light in an in an organic EL device.
In addition, the molecule represented by Formula (1) functions as a hole trapping unit, and the molecules represented by Formulas (2) and (3) function as a connecting unit for increasing the molecular weight. The molecule represented by Formula (2) functions as an electron trapping unit, and function to control the amount of the electrons flowing in the light-emitting layer even though the amount is small. Further, the molecule represented by Formula (4) is a functional group for avoiding the possibility of the end of the molecule to be a halogen atom.
Since the polymer molecules are capable of trapping holes, they can be used in a light-emitting layer of an organic EL device, to allow the polymer molecules themselves to trap holes flowing in the light-emitting layer. As a result, the polymer molecules generate cations. Then, these cations trap electrons flowing in the light-emitting layer, and thus recombination occurs within the polymer molecules. Thus, the molecule units represented by Formulas (5) to (7) etc. which are the light-emitting units (light-emitting molecules) perform EL light emission.
As used herein, the “recombination” means that a molecule in the light-emitting center is transferred into an excited state, which is derived by the holes and the electrons trapped by the polymer molecule.
That is, energy emitted during a process for transition from an “excited state” to a “ground state is observed as EL light emission.
According to the configuration described above, by using the polymer molecules in an organic EL device, light emission particularly with high efficiency and long life time, that is, light emission in a luminescent color region determined by a light-emitting molecule can be obtained.
Further, in the compound for an organic EL device, the polymer molecules are preferably represented by Formula (8).
(wherein A represents one selected from the groups represented by Formulas (9) to (11), R represents an alkyl group, an aryl group, or an alkylaryl group, and R″ represents hydrogen, an alkyl group, or an alkylaryl croup. Furthers m, n, and p each represent an integer of at least 1, and q and b each represent an integer of at least 0. r represents an integer of at least 1)

As described above, by using the polymer molecules in an organic EL device, light emission with high efficiency and long life time can be obtained.
In addition, in Formula (8), r represents an integer indicating a polymerization degree of an oligomer unit which constitutes the polymer molecule (which means a minimum unit constituted with the units represented by Formulas (7) to (9), and Formulas (1) to (4).
In Formula (8), q is preferably 0, but q may be an integer of 1 to 4 according to the types of the solvent used for a coating ink, since the solubility may be decreased in the absence of the constituent unit represented by Formula (3) or by r values.
Furthermore, for the oligomer unit represented by Formula (8) in the compound for an organic EL device, an integer m, which refers to the number of the units represented by A, is preferably 1 or 2. If there exist two or more light-emitting centers within the dopant molecule, a problem such as concentration quenching may occur, thereby m being preferably 1.
For the above-configured oligomer, r is 1 in Formula (8).
By such the molecular design of the oligomer, a sufficient EL light-emitting luminance can be obtained.
Further, for the oligomer unit represented by Formula (8) in the compound for an organic EL device, an integer n, which refers to the number of the units represented by Formula (1) as the units having a hole trapping function, is preferably at least 2.
By such the molecular design of the oligomer, a sufficient EL light-emitting luminance can be obtained.
In addition, for the oligomer unit represented by Formula (8) in the compound for an organic EL device, an integer p, which refers to the number of the units represented by Formula (2) as the units having an electron trapping function, as well as functioning as a connecting unit for increasing the molecular weight, is preferably 1 to 4.
By such the molecular design of the oligomer unit, the solubility in a solvent can be improved, and the flow of the electrons can be controlled, thereby promoting the optimization of the luminous efficiency.
Further, in the construction of the oligomer unit represented by Formula (8) in the compound for an organic EL device, it is preferable that the unit represented by A is directly connected to the unit represented by Formula (1) at one or more points. This is because the connection between the unit represented by A and the unit represented by Formula (1) has a significant effect on a fluorescence waveform, that is, an EL waveform.
Specifically, if a group represented by Formula (9) is used as the unit represented by A, it is preferable that the unit represented by Formula (9) is directly connected to the unit represented by Formula (1) at two points. If the unit represented by Formula (9) is directly connected to the unit represented by Formula (1) at one point, a yellow green color is emitted, whereas if the unit represented by Formula (9) is directly connected to the unit represented by Formula (1) at zero point, a blue green color is emitted.
Further, if a group represented by Formula (10) is used as the unit represented by A, it is preferable that the unit represented by Formula (10) is directly connected to the unit represented by Formula (1) at one or more points in order to emit yellow light.
In addition, if a group represented by Formula (11)) is used as the unit represented by A, it is preferable that the unit represented by Formula (11) is directly connected to the unit represented by Formula (1) at one or more points in order to emit green light.
By such the molecular design of the oligomer unit, a hole trapping property can be improved, thereby promoting the improvement of luminous efficiency, and luminance half life.
Further, for the configuration of the oligomer unit represented by Formula (8) in the compound for an organic EL device, it is preferable that an integer b, which refers to the number of the units represented by Formula (4) as a functional group for avoiding the possibility of the end of the molecule to be a halogen atom, is preferably 2.
In addition, the organic EL device according to an aspect of the invention includes the compound for an organic EL device.
By using the compound for an organic EL device, a good organic EL device is obtained.
Further, in the organic EL device, the compound for an organic EL device is preferably used in the light-emitting layer.
By using the compound for an organic EL device in the light-emitting layer, the characteristics of the material can be exhibited. Based on this, it is possible to obtain a device having good characteristics such as luminous efficiency and luminance half life.
Further, in the organic EL device, the compound for an organic EL device is preferably used as a light-emitting dopant material in the light-emitting layer.
As used herein, the light-emitting layer refers to a region (layer) which performs EL light emission while applying a voltage to generate a current. In the case of a coated organic EL, many of the materials constituting the light-emitting layer are typically composed of one kind thereof only. It has three functions, that is, an EL light-emitting function, in addition to electrons and holes injecting/transporting functions.
Further, the light-emitting dopant refers to a dopant which is used in a light-emitting layer, and has a main function to emit lights among the above-mentioned three functions. At this time, an organic material having main functions to inject/transport electrons and holes is used at the same time, and this material is referred to a host material.
By using the compound for an organic EL device as a light-emitting dopant material in the light-emitting layer, the characteristics of the material can be exhibited. Based on this, it is possible to obtain a device having good characteristics such as luminous efficiency and luminance half life.
Further, in the organic EL device, the light-emitting layer is formed of a light-emitting dopant material and a host material, wherein the light-emitting dopant material and the host material in the light-emitting layer are preferably contained at such a ratio that k expressed in % by weight as calculated by Equation 12, is 0.5% by weight or more and 10.0% by weight or less.k=(a/(b+c))×100  Equation 12(wherein a is the weight of the units containing the light-emitting molecules in the light-emitting dopant material, b is the weight of the light-emitting dopant material used, and c is the weight of the host material used)
Generally, the light-emitting region (the light-emitting molecule, in the case of the light-emitting dopant material) in the light-emitting dopant has a high fluorescence intensity. As a result, it is said that if the light-emitting dopant in the device (element) can trap electrons and holes with high efficiency, it is possible to induce EL light emission even when the k value is around 0.1% by weight. However, if the k value is too small, there are problems such as light emission of the host material due to insufficient energy transfer, or insufficient electron and hole trapping. Accordingly, the lower limit of the range of the k values is preferably set at 0.5% by weight. Further, it is difficult to define the upper limit of the range of the k values, and as long as the light-emitting dopant is intended to exhibit a light-emitting function, the upper limit of the range of the k values may be set at from 20% by weight to 30% by weight. However, if the addition amount is too high, sufficient EL light emission cannot be obtained due to concentration quenching. Therefore, the realizable upper limit of the range of the k values for luminous efficiency is preferably set at 10% by weight.
Further, in the organic EL device, the light-emitting layer is formed of a light-emitting dopant material and a host material, and the host material is a homopolymer or copolymer containing at least one material backbone selected from fluorene, arylamine, and anthracene.
The host material is required to have a performance to transport holes and electrons well. Further, the energy gap between a HOMO (highest occupied molecular orbital) and a LUMO (lowest unoccupied molecular orbital) in a molecule orbital is required to be larger than those of the units (light-emitting molecules) represented by Formulas (5) to (7). The vacuum level of the LUMO (lowest unoccupied molecule orbital) is required to be larger than that of the polymer represented by Formula (8).
By satisfying the above-mentioned requirements, both holes and electrons are favorably injected from the host material to the light-emitting dopant, thereby improving luminous efficiency and life time.
Further, in the organic EL device, at least one layer of a hole injecting layer or a hole transporting layer is preferably provided between the light-emitting layer and the anode.
By such the configuration, holes are more easily injected to the light-emitting layer, thereby promoting the improvement of luminous efficiency.
Further, the HOMO (highest occupied molecular orbital) can be measured by means of a photoelectron spectroscopic device (AC-1) manufactured by Riken Keiki Co., Ltd., etc.
Further, the energy gap between the HOMO (highest occupied molecular orbital) and the LUMO (lowest unoccupied molecular orbital) can be measured by means of a thin film absorption spectrum of the host material used, and thus generally, an absorption band at a largest wavelength of the absorption spectrum is used as an energy gap. Further, the vacuum level of the LUMO can be determined more simply, as compared with the vacuum level of HOMO, and the gap of HOMO-LUMO.
Further, in the organic EL device, the light-emitting layer is preferably prepared by application using a spin coating method or a liquid droplet discharge method.
As such, the compound for an organic EL device is applied using a spin coating method or a liquid droplet discharge method to prepare a light-emitting layer. As a result, it is possible to obtain a device having good characteristics such as luminous efficiency and luminance half life.
Here, since the compound for an organic EL device is an oligomer or a polymer molecule, and it has good compatibility with the polymer of the host material. Accordingly, the host material and the light-emitting dopant material can be uniformly dispersed in the light-emitting layer.
Further, since the compound for an organic EL device has a large molecular weight, a part or all of the compound is decomposed during deposition. As a result, the characteristics of the obtained organic EL device are deteriorated.